1. Field of the Invention
The present invention relates to a method of processing seismic data. In another aspect, the present invention relates to a method for processing seismic data having multiple reflection noise. In still yet another aspect, the present invention relates to a method for isolating multiple reflection noise in seismic data.
2. Description of the Related Art
Seismic exploration generally involves generating seismic pulses at the surface of the earth by means of one or more seismic sources. The seismic pulses travel downwardly into the earth with a fractional amount being reflected and/or refracted due to differences in elastic properties at the interface of various subterranean formations.
Detectors, such as seismometers, geophones or hydrophones, produce analog electrical seismic signals or seismic trace signals in response to detected seismic wave reflections and/or refractions. The analog electric seismic signals or seismic trace signals from the detectors can then be recorded. Alternatively, the analog seismic signals or seismic trace signals from the detectors can be sampled and digitized prior to being recorded. The seismic data recorded in either manner are subsequently processed and analyzed to determine the nature and structure of the subterranean formations.
From the recorded data, a seismic section is generated. A seismic section is a seismic image depicting the subsurface layering of a section of earth along a seismic line of profile. The seismic section is an important tool which the geologist studies to determine the nature of the earth's subsurface formations. However, before an array of seismic samples can be converted into a seismic section which can be interpreted by the geologist, the seismic data must be processed to reduce the degradation due to noise.
Seismic interpretation generally involves the study of the behavior of arrival times, amplitudes, velocities, frequencies, and character of the reflections from target horizons. Any changing or anomalous behavior is of particular interest.
Multiple reflection energy, commonly known as "multiples", are a well known geophysical phenomenon that are generally defined as seismic energy which has been reflected more than once. Multiples are commonly generated in a layer which is bounded by layers of much different density, for example, water bounded by the water bottom and the water surface. Thus, water bottoms tend to be a common source for multiples, although multiples can occur in land data also.
Generation of water bottom multiples occurs when a portion of a seismic signal travelling through the water is reflected off of the water bottom. This reflected signal then travels within the water layer toward the water surface where it is reflected off of the water surface back toward the water bottom. The reflection between the water bottom and the water surface may occur one or more times. As the seismic receivers merely measure arrival time without regard for travel path, seismic signals which have undergone a tortuous travel path appear deeper in the seismic record than they physically are in the subsurface.
Traditional prior art methods for removing multiples from seismic data, such as predictive deconvolution, generally have utilized, and required, periodicity of the multiples for proper processing. That is, after the first occurrence the multiple would repeat every n milliseconds in the trace. For example, in predictive deconvolution, information from the earlier part of the seismic trace is used to predict and deconvolve the latter part of the trace.
Other traditional art methods, such as normal moveout, require a significant velocity difference between primaries and multiples. For example, to the extent that long-path multiples travel at a lower average velocity than primary reflections for the same arrival time, they will show greater normal moveout and can be attenuated with common-depth-point stacking.
Unfortunately, these very assumptions of periodicity or significant velocity differences tend to limit the application and/or effectiveness of predictive deconvolution or normal moveout techniques.
Another art method utilized to eliminate the effects of multiples include the wave-equation method which results in a more rigorous treatment of predicting the multiple traveltimes. The wave-equation method can handle complex geometry and requires no knowledge of the velocity of the subsurface. Unfortunately, the computing cost associated with the wave-equation method has severely limited its use.
Thus there is a need in the prior art for an alternative method of processing seismic data containing multiples.
There is another need in the art for an improved method of processing seismic data containing multiples that will overcome the limitations of the art methods.
There is still another need in the prior art for an improved method of processing seismic data containing multiple noise that is not dependent upon the periodicity of the data.
There is even another need in the prior art for an improved method of processing seismic data containing multiple noise that is not dependent upon a large velocity difference between a primary event and its multiples.
There is even still another need in the prior art for an improved method of processing seismic data containing multiple noise that is not overly computationally expensive.
These and other needs of the art will become apparent to those of skill in the art upon review of this patent specification.